Perimenopause is one of the most complex and least understood states of a woman's life. Many of the health risks associated with this state were believed to be due to decreases in estrogen levels and that estrogen could protect against health risks faced by perimenopausal women. However, estrogen fluctuates during perimenopause and if at all increases during the premenstrual and follicular phases. Recent clinical trials have shown that chronic administration of estrogenic compounds in postmenopausal women may increase the risk for several diseases. Therefore, it is important to investigate the effects of estrogen exposure on various organ systems. Studies so far indicate that estrogen's effects in the brain are beneficial. These reported effects, however, deal with non-hypothalamic regions of the brain. The effects of chronic estrogen exposure on the hypothalamus which regulates several key body functions have not been investigated. This is critical because women use estrogenic preparations on a prolonged basis and are exposed to endogenous estrogen throughout their adulthood. This proposal focuses on the effects of chronic estrogen exposure on one of the estrogen sensitive neuronal systems of the hypothalamus, namely, the tuberoinfundibular dopaminergic (TIDA) system. Dopamine (DA) released from TIDA neurons inhibits prolactin (PRL) secretion from the anterior pituitary. Age-related reductions in TIDA activity is associated with hyperprolactinemia and appearance of mammary and pitutiary tumors in animal models. The mechanisms behind the loss of TIDA neuronal function is not clear. In this application, we are proposing a novel hypothesis and an interesting model to study how estrogen could affect TIDA neurons and increase PRL levels. This series of studies is important because women not only use estrogen on a long-term basis in HRT but are also exposed to environmental estrogens. Prolonged exposure to estrogen and elevated levels of PRL may promote the risk for breast cancer.